[messaging] Giving new devices access to old messages

Matthew Hodgson matthew at matrix.org
Sat Apr 18 18:50:59 PDT 2015

On 19/04/2015 01:22, Trevor Perrin wrote:
> So here's another approach, which I'll explain by analogy to the Tahoe
> distributed filesystem [1]:  In Tahoe clients can exchange "read
> capabilities" or "read-caps" which grant access to a file stored
> elsewhere.  A read-cap contains a symmetric encryption key for an
> encrypted file and a secure hash of its ciphertext, so it has all the
> data needed to retrieve and decrypt an authentic copy of the file.

How does this differ from caching per-message PFS keys on the receiving 
device, and sharing them with the new device when migrating?

This is the current plan for Matrix, albeit allowing conversations to 
configure coarser PFS granularity if desired (effectively a shared key 
for certain sets of messages, grouped by time-range or message-type 
etc).  This gives the option of reducing the number of keys that need to 
be stored on multiple devices in order to decrypt server-side message 
history - at the (optional) expense of pure PFS, of course.

Apologies if I'm missing the subtlety here :)


Matthew Hodgson

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